Abstract A ruling hypothesis for the central Cascade Range in Washington is that the Eocene arkosic formations, which are kilometers thick, were deposited in local grabens, such as the Chumstick Formation in the putative Chiwaukum graben. However, the formations are regional in extent and are preserved in less extensive northwesttrending synclines. The Chumstick Formation in the Peshastin syncline is a more proximal equivalent of the Roslyn Formation, which is preserved in the Kittitas Valley syncline 25 km to the southwest. The Chiwaukum structural low is partially bounded on the southwest by the Leavenworth fault zone, which consists of northwesterly striking, northeasterly verging reverse faults (with associated northwest-striking folds). The reverse faults and the hinges of the folds are cut by N-S, dextral strike-slip faults, which also partially bound the Chiwaukum structural low. Conglomeratic units in the Chumstick Formation are not proximal to either set of bounding faults. The Leavenworth fault occurs on the steeper northeastern limb of a northwesterly trending, basement-cored anticline. The Eagle Creek and Ainsley Canyon anticlines also have reverse faults on their steeper northeastern limbs. In the Puget Lowland, the Seattle reverse fault is in a similar anticline. The regional distribution of the Eocene formations and uplift of the Cascade Range are caused by folding of the Miocene Columbia River Basalt Group since 4 Ma. The remnant of a 4 Ma andesite on Natapoc Mountain shows that the present low topography of the Chiwaukum structural low is erosional and young.

Abstract A prevailing hypothesis for the central Cascade Range of Washington State is that it underwent regional extension or transtension during the Eocene. This hypothesis is based on the idea that kilometers-thick, clastic, Eocene formations were deposited syntectonically in local basins. Our mapping and structural analysis indicate that these formations are preserved in fault-bounded, regional synclines, not in separate depositional basins. Thus, the type area for the hypothesis, the so-called Chiwaukum graben, is here renamed the Chiwaukum Structural Low. The Eocene arkosic Chum-stick Formation, which was thought to have been syntectonically deposited in the graben, is the proximal equivalent of the Roslyn Formation 25 km southwest of the graben. Because the name “Roslyn Formation” has precedence, the name “Chumstick Formation” should be abandoned. Additionally, several areas previously mapped as Chumstick Formation in the Chiwaukum Structural Low probably are parts of the older Swauk Formation and younger Wenatchee Formation. The southwestern boundary of the Chiwaukum Structural Low includes the Leav-enworth fault zone, which consists of postdepositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive early-Eocene, arkosic Swauk Formation over the mid-Eocene, arkosic Chumstick Formation. A diamictite, which previously was placed in the Chumstick Formation and inferred to have been syntectonically derived from the Leavenworth fault zone, is part of the older Swauk Formation. We mapped a 0.6–1-km-thick conglomerate-bearing sandstone as a robust marker unit in the Chumstick Formation; instead of being spatially related to the bounding faults, this unit has a >30 km strike length around the limbs of folds in the structural low. The northwest-striking reverse faults and fold hinges of the structural low are cut by north-striking strike-slip faults, which likely are late Eocene to Oligocene; these north-south faults partially bound the structural low. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group; this younger folding largely defines the regional map pattern, including the structural low. A model to account for the above characteristics is that all of the Eocene formations, not just the Roslyn Formation, are kilometers thick and are remnants of regional unconformity-bounded sequences that were deposited on the Eocene margin of this part of North America. Their present distribution is governed by younger faults, folds, and erosion. Thus, the Eocene to Recent history of the central Cascade region is characterized not by crustal extension, but by episodes of folding (with related reverse faults) and strike-slip faulting.

Abstract Eocene sedimentary and volcanic rocks on the eastern flank of the Cascade Range consist of five regional, unconformity-bounded formations of the Challis synthem. These formations define a series of northwesterly striking folds. Five anticlines are 9 to 28 km apart, have pre-Tertiary crystalline rocks in their cores, high-angle reverse faults on their steeper northeastern limbs, and pass down-plunge into more gentle folds in the Neogene Columbia River Basalt Group (CRBG). Such northwesterly trending folds extend from east of the Columbia River across the Cascade Range to the Puget Lowland. The Chiwaukum graben and Swauk basin, which heretofore were thought to be local, extensional, depositional basins, are, instead, the major northwesterly trending synclines in this series of folds. The Eocene formations were preserved, not deposited, in these synclines. Dextral, N-S faults cut the reverse faults and the pre-CRBG portion of some of the folds. The post-CRBG folds control the regional distribution of the Eocene formations. The Cascade Range is a southerly plunging, post-CRBG anticline. Clasts in the Thorp Gravel indicate that this anticline began to rise ca. 4 Ma. The anticline has an amplitude of ∼3.5 km, and it causes the plunges of the northwesterly striking post-CRBG folds. The northerly and northwesterly post-CRBG folds form a regional interference pattern, or “egg-crate,” that dominates the present topography of Washington State.